1. Field of the Invention
The present invention relates, generally to an automatic transmission and, more specifically, to an automatic transmission having hydraulic valves with flow force compensation.
2. Description of the Related Art
Generally speaking, land vehicles require a powertrain consisting of three basic components. These components include a power plant (such as an internal combustion engine), a power transmission, and wheels. The power transmission component is typically referred to simply as the “transmission.” Engine torque and speed are converted in the transmission in accordance with the tractive-power demand of the vehicle.
Automated manual type transmissions can be power-shifted to permit gearshifts to be made under load. One type of automated manual transmissions (AMT) has two clutches and is generally referred to simply as dual, or twin, clutch transmissions (DCTs). The dual clutch structure is most often coaxially and cooperatively configured to derive power input from a single engine flywheel arrangement. However, some designs have a dual clutch assembly that is coaxial, but with the clutches located on opposite sides of the transmissions body and having different input sources. Regardless, dual clutch transmissions typically include one power transmission assembly on each of two input shafts concomitantly driving one output shaft. Each clutch and associated gear sets can be shifted and clutched independently. In this manner, uninterrupted power upshifting and downshifting between gears, along with the high mechanical efficiency of a manual transmission is available in an automatic transmission form. Thus, significant increases in fuel economy and vehicle performance may be achieved through the effective use of certain automated manual transmissions.
The dual clutch transmission structure may include two dry disc clutches each with their own clutch actuator to control the engagement and disengagement of the two-clutch discs independently. While the clutch actuators may be of the electromechanical type, since a lubrication system within the transmission requires a pump, some dual clutch transmissions utilize hydraulic shifting and clutch control. Shifts are accomplished by engaging the desired gear prior to a shift event and subsequently engaging the corresponding clutch. With two clutches and two inputs shafts, at certain times, the dual clutch transmission may be in two different gear ratios at once, but only one clutch will be engaged and transmitting power at any given moment. To shift to the next higher gear, first the desired gears on the input shaft of the non-driven clutch assembly are engaged, then the driven clutch is released and the non-driven clutch is engaged.
This requires that the dual clutch transmission be configured to have the forward gear ratios alternatingly arranged on their respective input shafts. In other words, to perform up-shifts from first to second gear, the first and second gears must be on different input shafts. Therefore, the odd gears will be associated with one input shaft and the even gears will be associated with the other input shaft. In view of this convention, the input shafts are generally referred to as the odd and even shafts. Typically, the input shafts transfer the applied torque to a single counter shaft, which includes mating gears to the input shaft gears. The mating gears of the counter shaft are in constant mesh with the gears on the input shafts. The counter shaft also includes an output gear that is meshingly engaged to a gear on the output shaft. Thus, the input torque from the engine is transferred from one of the clutches to an input shaft, through a gear set to the counter shaft and from the counter shaft to the output shaft.
Gear selection and gear engagement in either an AMT or a DCT is similar to that in a conventional manual transmission. One of the gears in each of the gear sets is disposed on its respective shaft in such a manner so that it can freewheel about the shaft. A synchronizer is also disposed on the shaft next to the freewheeling gear so that the synchronizer can selectively engage the gear to the shaft. The majority of the newer AMT and DCT designs employ 6 forward gears and a reverse gear, which provides greater efficiency and fuel economy by having closer ratio gear sets than previous designs.
While automated manual and dual clutch transmission have overcome several drawbacks associated with conventional transmissions, it has been found that controlling and regulating these automatically actuated transmissions to achieve the desired vehicle occupant comfort goals in an efficient and cost effective manner is a complicated matter. There are a large number of events to properly time and execute within the transmission for each shift to occur smoothly and efficiently.
Furthermore, since the control of these types of automatic transmissions is carried out by hydraulically actuating the various components within the transmission, it is important to provide a stable hydraulic pressure. Since hydraulically actuated devices respond in a predetermined and a precise manner for the given pressure supplied to actuate them, inaccurate control of the hydraulic pressure causes inaccurate operation and control of a AMT or DCT transmission. Up to this point, establishing and maintaining a stable hydraulic pressure in these newer types of automatic transmissions has proven problematic. As previously mentioned, a pump is employed to provide pressurized hydraulic fluid for the control and actuation of the transmission. In addition, the clutches and gear assemblies are lubricated and cooled by a secondary flow of hydraulic fluid. Typically, the pump is mechanically driven by a power take-off from the engine. Thus, the hydraulic pressure delivered from the pump increases as the pump speed increases in response to an increase in engine speed.
To address the changes in the hydraulic pressure delivered by the pump as engine speed changes, the hydraulic supply circuits of conventional dual clutch transmissions include a plurality of hydraulic valves. One of the functions of these valves is to establish and maintain a specific predetermined pressure in the hydraulic line. Valves of the type typically employed in the hydraulic circuit of a transmission usually include a valve member slideably disposed within a valve body that moves back and forth over the various ports in the valve body to direct and control the fluid flow between the ports.
Since the pump is sized to provide the necessary pressure at idle and provides increased pressure as the engine speed increases, many of the hydraulic valves are designed to dump, or bleed off the excessive flow to the return, or suction side of the pump. This action provides, at best, a rudimentary regulation of gross variations in pressure. However, the design and operation of these valves typically fail to properly account for various flow effects of the hydraulic fluid within the hydraulic circuit and do not provide the precise and stable hydraulic pressure that is necessary to ensure accurate control over the AMT or DCT transmission. More specifically, to provide a stable system pressure, the hydraulic valves must be responsive to changes in the flow forces that occur within the circuit due to changes in the hydraulic flow in the line pressure side and the return, or suction side of the valve.
The flow force is the relative force of the hydraulic fluid that acts upon the lands of the valve member as the fluid moves through the valve. Flow forces may be either steady state or transient. Steady state flow force is the force of the hydraulic fluid upon the valve member that results from the fluid accelerating property of the orifice formed within the valve between the valve member and the inlet and outlet ports. The steady state flow force is directly proportional to the pressure drop through the valve and the area of the formed orifice. The steady state flow force always acts in a direction to close the valve. Steady state flow forces relate to a steady state of the valve member due to relatively constant flow conditions.
Transient flow force is the force in the hydraulic fluid that occurs when the valve member is moved and is due to the change of speed of the fluid moving through the valve as the size of the flow area within the valve changes. The magnitude of transient flow force is proportional to the velocity of the movement of the valve member and pressure changes. The direction of the transient flow force depends on change to the flow. Transient flow forces relate to the movements of the valve member.
The effects of these flow forces upon the valves are manifest as the fluid flow moves through the valve body. As the hydraulic fluid moves through the valves, the inherent flow forces act against the physical surfaces of the valve member, and the applied force can physically effect the position of the valve member in the valve body causing it to move and generate instability in the valves. For example, an increase in fluid flow from the pump may act upon the valve member surfaces forcing them open further, or an increase in pump suction may cause the valve member to move in an uncontrolled manner. The movement of the valve member caused by the flow forces results in instability in the line pressure and causes further variations in the flow as the valve member tries to correct.
The structural configuration of valves used in conventional transmissions is generally one of two known types. Unfortunately, neither is without certain drawbacks. These two types of valve configurations are based upon their different approaches to how the hydraulic pressure is physically “metered.” One approach involves valve member and port interaction that is known as a “meter-in” configuration, in which the valve member is designed to move across and meter the hydraulic pressure at the inlet port with the return or suction port of the valve open and unrestricted. A meter-in configuration provides good control over the steady state flow but is generally unstable in regulating transient flow force. The other valve design approach is known as a “meter-out” configuration. With a meter-out configuration, the valve member is designed to move across and meter the hydraulic pressure on the suction (outlet) port with the inlet port of the valve open and unrestricted. A meter-out configuration provides good control during transient flow force conditions, but offers less stable control of the steady state flow force. The lack of valve stability in either of these configurations introduces line pressure fluctuations and subsequent inaccurate actuation and control of the dual clutch transmission. The inefficiencies and inaccuracies in hydraulic control of the dual clutch transmission that are attributable to the hydraulic valves are distinct and produce quantifiable losses of vehicle output power and fuel economy. Thus, while the current valves used to control the hydraulic pressure in a dual clutch transmission have generally worked for the intended purpose, they are still susceptible to flow forces fluctuations causing inaccurate hydraulic control of the transmission.
In an AMT or DCT transmission, the hydraulic valves are most often subject to steady state flow forces, while transient flow forces occur only during certain operational periods. Therefore, even with the above-mentioned instabilities, the meter-in configuration is the most common design type employed in conventional dual clutch transmissions. Attempts have been made to compensate for the transient flow force effects that act on valve members with meter-in configuration. However, these attempts have been largely unsuccessful and have only made compensations for transient flow force effects at the expense of introducing instabilities in steady state flow force control. Thus, the conventional approaches employed with hydraulic valves in an AMT or DCT transmission remain inefficient and susceptible to fluctuations and inaccurate control of the hydraulic pressure causing inaccurate hydraulic control of the dual clutch transmission. Accordingly, there remains a need in the related art for an automatic transmission having a hydraulic valve with flow force compensation that provides stable hydraulic pressure for both steady state flow and transient flow conditions.